1. Field of the Invention
The invention relates to the methods for making molybdenum compounds and more particularly relates to methods for making molybdenum alcohol complexes useful as olefin epoxidation catalysts.
2. Other Related Methods in the Field
It is well known that the epoxidation of olefins to give various oxide compounds has long been an area of study by those skilled in the art. It is equally well known that the reactivities of the various olefins differs with the number of substituents on the carbon atoms involved in the double bond. Ethylene itself has the lowest relative rate of epoxidation, with propylene and other alpha olefins being the next slowest. Compounds of the formula R.sub.2 C.dbd.CR.sub.2, where R simply represents alkyl or other substituents, may be epoxidized fastest. Thus, the more substituents on the double bond carbons, the easier it is to epoxidize across that bond.
Of course, the production of ethylene oxide from ethylene has long been known to be accomplished by reaction with molecular oxygen over a silver catalyst. Numerous patents have issued on various silver-catalyzed processes for the production of ethylene oxide. Unfortunately, the silver catalyst route will not work for olefins other than ethylene. For a long time the commercial production of propylene oxide could only be accomplished via the cumbersome chlorohydrin process.
Another commercial process for the manufacture of substituted oxides from alpha olefins such as propylene was not discovered until John Kollar's work in the 1960s. His U.S. Pat. No. 3,351,635 taught that an organic oxide compound could be made by reacting an olefinically unsaturated compound with an organic hydroperoxide in the presence of a molybdenum, tungsten, titanium, columbium, tantalum, rhenium, selenium, chromium, zirconium, tellurium or uranium catalyst. Kollar's U.S. Pat. No. 3,350,422 teaches a similar process using a soluble vanadium catalyst.
However, even though Kollar's work was recognized as extremely important in the development of a commercial propylene oxide process that did not depend on the chlorohydrin route, it has been recognized that Kollar's catalytic route (in which molybdenum is the preferred catalyst) has a number of problems. For example, large quantities of the alcohol corresponding to the peroxide used were formed; if t-butyl hydroperoxide was used as a co-reactant, then a use or market for t-butyl alcohol had to be found. Other troublesome by-products were the olefin oligomers. If propylene was the olefin to be epoxidized, various propylene dimers, sometimes called hexenes, would result. Besides being undesirable in that the best use of propylene was not made, problems would be caused in separating the desired propylene oxide from the product mix. In addition, the molybdenum catalyst may not be stable or the recovery of the catalyst for recycle may be poor.
Various avenues of investigation have been explored in attempts to improve on the molybdenum-catalyzed epoxidation of propylene. One technique was to try to improve on the catalyst itself. Patents which cover the preparation of various molybdenum epoxidation catalysts include U.S. Pat. No. 3,362,972 to Kollar. There a hydrocarbon soluble salt of molybdenum or vanadium may be made by heating a molybdenum compound in which molybdenum has a valence of +6, or a vanadium compound in which vanadium has a valence of +5, with a carboxylic acid of from 4 to 50 carbon atoms having at least 4 carbon atoms per carboxylic group. U.S. Pat. No. 3,578,690 to Becker discloses that molybdenum acid salts may be made by directly reacting a carboxylic acid with a molybdenum compound while removing the water that is formed.
The reaction of molybdenum trioxide with monohydric saturated alcohols having 4 to 22 carbon atoms or with a mono- or polyalkylene glycol monoalkyl ether or mixtures thereof to make olefin epoxidation catalysts is described in U.S. Pat. No. 3,480,563 to Bonetti, et al. These catalysts have only 0.07 to 0.93% molybdenum, which is a molybdenum content too low for commercial use. Bonetti, et al. do not realize the importance of the ratio of alcohol to molybdenum compound reactants with respect to maximizing molybdenum content yet providing a soluble, active epoxidation catalyst. They also do not indicate any benefit from adding ammonium hydroxide to the preparation, an important factor discovered when molybdenum trioxide is reacted with 2-ethyl-1-hexanol.
In U.S. Pat. No. 4,434,975 to ARCO, investigators found that molybdenum catalysts could be made from saturated alcohols or glycols having one to four carbon atoms, such as ethylene glycol and propylene glycol, by reacting them with molybdenum metal and an organic hydroperoxide, peroxide, or H.sub.2 O.sub.2. Molybdenum compounds prepared by reacting an ammonium-containing molybdate with a hydroxy compound, for example, an organic primary or secondary alcohol, a glycol or a phenol, are described in U.S. Pat. Nos. 3,784,482 and 3,787,329 to Cavitt.
Further, U.S. Pat. No. 3,573,226 to Sorgenti discloses that molybdenum-containing epoxidation catalyst solutions may be made by heating molybdenum powder with a stream containing unreacted tertiary butyl hydroperoxide and polyhydric compounds of from about 200 to 300 molecular weight and having from 4 to 6 hydroxyl groups per molecule. These catalysts are used for the epoxidation of propylene according to U.S. Pat. No. 3,666,777 to Sorgenti.
U.S. Pat. No. 3,953,362 to Lines, et al. reveals that novel molybdenum epoxidation catalysts may be prepared by reacting an oxygen-containing molybdenum compound with hydrogen peroxide and an amine and optionally water or an alkylene glycol at elevated temperatures. Similar catalysts are prepared by reacting an oxygen-containing molybdenum compound with an amine and an alkylene glycol at elevated temperatures according to U.S. Pat. No. 4,009,122 also to Lines, et al.
U.S. Patent to Mattucci, et al. also concerns molybdenum glycol catalysts prepared from molybdenum acetyl acetonate and isolated as solids. When the materials are used as epoxidation catalysts, they must be employed in solution with a hydrocarbon solvent. Molybdenum derivative compounds also useful as epoxidation catalysts may be prepared by reacting an oxygen-containing molybdenum compound such as molybdenum acetylacetonate, molybdic acids and molybdenum oxides with an organic compound having vicinal hydroxyl groups in the presence of a hydrohalic acid such as hydrofluoric acid, hydrochloric acid and the like, according to U.S. Pat. No. 3,991,090 to Hagstrom, et al.
There still exists a need for an epoxidation catalyst that is stable, easy to prepare, and has a high molybdenum content. Recent research also suggests that such a catalyst should also be nonacidic from the standpoint of not containing any excess or free carboxylic acid (solvent) if the catalyst involves reaction of molybdenum trioxide or ammonium heptamolybdate with a carboxylic acid.